Diagnosing & fixing feedback issues

Discussion in 'Live Sound [BG]' started by wemmick, Dec 8, 2012.


  1. wemmick

    wemmick Supporting Member

    Joined:
    May 23, 2010
    Location:
    Washington, DC
    I'm looking for some guidance in diagnosing and correcting feedback issues. We played a local bar last weekend and when one of our six vocal mics was turned up, we got lots of feedback in our monitors. We're an amateur band and I'm the de facto sound guy, but I've got no training in this.

    Our PA setup is pretty basic -- Peavey unpowered mixer, a pair of EV ELX115P for mains and a pair of Behringer B212Ds for monitors. Only vocals are going through the PA. Drums are unamplified, guitars, keys and bass run their own amps.

    One mic was back with the drummer, the other five were more or less lined up across the stage. The one that gave trouble was on the far left side of the stage -- near one speaker. The house didn't hear any feedback issues, but we heard them through the monitors.

    Questions:

    • Why would one mic be pickier than the others?
    • Why would it feedback only when being sung into -- if it were running too hot, wouldn't the ambient noise it's picking up trigger feedback without somebody directly singing into it?
    • What can I do to combat feedback with the equipment we've got? (I'm very much an amateur sound guy)
    • Does anybody know of good online resources for learning more about general PA setup & tuning? (because I'm frustrated that I don't know enough about this stuff)
  2. seamonkey

    seamonkey

    Joined:
    Aug 6, 2004
    Feedback eliminator, feedback ferret, ...
    They work
    Get to the club early so you have time to run the automatic setup a few times with different mic placement.
  3. testing1two

    testing1two Gold Supporting Member

    Joined:
    Feb 25, 2009
    Location:
    Southern California
    mic placement, mic placement, mic placement.

    Using a graphic or parametric EQ is certainly important and will give you more headroom before feedback by notching out specific offending frequencies but you cannot escape the laws of mic placement.
  4. ggunn

    ggunn

    Joined:
    Aug 30, 2006
    Location:
    Austin, TX
    Different mics can have different susceptibility to feedback. The problem one in your setup may have a lower threshold or a wider pattern. If it's a condenser mic, switch it out for a dynamic. If it's an omnidirectional mic, switch it for a cardioid or supercardioid.

    It also may be in a hot spot on stage where because of speaker aiming and/or room acoustics it "hears" the monitors more than the others. Or its trim pot on the board may be set higher.

    Make sure your mics are all behind a line drawn between the backs of your main speakers. Feedback eliminators supposedly work, though I've never used one. A 31 band EQ on the monitors will almost certainly help.

    It's not unusual for a mic to seem feedback free until someone uses it. It may be that the vocalist's voice triggers the oscillation, or it could be that the sound reflection from the vocalist's face sets it off. And don't let anyone grab a mic up near the end; a hand cupped around the mic cartridge will create a resonance chamber that can cause feedback.

    Good luck.
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  6. TimmyP

    TimmyP

    Joined:
    Nov 4, 2003
    Location:
    Indianapolis, IN
    Keep in mind when ringing out monitors, you must duplicate the performance situation. Ringing out with an open stage (turning the level up until it rings, then EQing the ring away) may or may not apply to the performance situation. In a multi-mic situation, where the noise-makers (mouths and instruments) are using the mic at near 0 distance, ringing an open stage gets few if any of the frequencies that will feed back when a hand, face, open mouth, or reflective instrument top (or even a hat brim) are in front of the mic. In the case of the one mic bluegrass performance, the open stage is pretty close to the performance situation. But don't forget to add the missing bits: the reflective guitar top that will bounce sound from the room or monitor into the mic, the hat brim, or - ? Even once you seem to be done, when soundcheck starts you may get additional "hot" or "loose" (not feeding back, but trying to) frequencies that need attention. (Always end the soundcheck a little loud, to hunt for such spots.)
  7. will33

    will33

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    austin,tx
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    Were you using all the same type of vocal mic, like all 58's or whatever? If so, it's easier to find feedback and bring it under control.

    +1 to placement and gain (trim) settings. The more bands of EQ available, the better. You can better chop out the offending frequency(s) without killing too much of the other sound.

    Sometimes it's difficult. Try angling the mic and or monitor some to help it out. Sometimes it can be a real problem child and you're pressed for time, so you just cut sliders until you find the right one, and just deal with whatever less-than-stellar tone that brings. Got to be flexible.
  8. will33

    will33

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    +1 to ringing it out a little hotter than you need to run it....gives a margin of error.
  9. lowfreq33

    lowfreq33

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    Jan 27, 2010
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    Nashville
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    Do you have a smartphone? There are a bunch of free RTA apps that will show you the frequency plot in real time.
  10. wemmick

    wemmick Supporting Member

    Joined:
    May 23, 2010
    Location:
    Washington, DC
    Yeah, I have an iPhone. What's RTA? real time analyzer?

    Is there an app you have liked?
  11. Jerry Ziarko

    Jerry Ziarko Supporting Member

    Joined:
    Feb 23, 2003
    Location:
    Rochester, NY
    Remember, work with the nulls. Cardioid mics like the monitor placed directly in front of the mic, while hyper-cardioid mics work best with the monitor at 110 degrees.(think 10 or 2 o'clock) ;)
  12. Kmonk

    Kmonk

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    Oct 18, 2012
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    South Shore, Massachusetts
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    Endorsing Artist: Fender and Spector, Ampeg, Curt Mangan Strings
    Type of mic, placement of mic, eq and volume.
  13. ggunn

    ggunn

    Joined:
    Aug 30, 2006
    Location:
    Austin, TX
    More than anything else, keep at it, and continue doing the things that work and stop doing the things that don't work. Experience is the only true teacher.

    I'm feeling very zen tonight. Do, or do not. There is no try. :D

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